I've never used Plex at all. This video on YouTube of the Roku interface looks similar to the Boxee Box:
http://www.youtube.com/watch?v=X8OYjkSyaxU

I'm sure it's crappy compared to the full-blown Plex desktop application, but this looks very workable. I'm interested in checking it out. I'm just going to wait for a hot deal on the Roku XS.
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-Rob Riccardelli
80GB 16MB MK2 090000736